This invention relates to regenerators including regenerators for regenerative gas cycle machinery.
Regenerative gas cycle machines are a class machinery that includes Stirling cycle engines and Stirling cycle, Gifford-McMahon and pulse tube refrigerators. A regenerator is a critical component of all regenerative gas-cycle machines. In theory, a parallel-plate configuration offers a more favorable relationship between heat transfer and pressure drop than any other regenerator configuration, maximizing effectiveness. To make a parallel plate regenerator with the tight flow passages required for service in regenerative gas cycle machinery, spaced layers of foil have been tried. In practice, performance of foil regenerators has been disappointing. In part, that disappointing performance is due to difficulty in creating and assembling foil regenerators with uniform flow channels.
Stirling cycle machines, including both engines and refrigerators, have been constructed with annular regenerators surrounding the cylinder housing the displacer. Those regenerators have been constructed with a continuous spiral wrap of solid metal foil using dimples in the metal to separate the layers from each other. However, because it is difficult to create dimples of uniform depth and because there can be no cross-flow through the solid foil to adjust pressure differences between different layers, uniform flow patterns have not been achieved and performance of foil regenerators has been limited.
Attempts have been made to fashion regenerators from foil layers separated by spacers such as strings or layers of photoresist material. Those efforts have not proven to be satisfactory. Photoresist tends to flake off of the foil and be carried to other parts of the machine, contaminating it. Strings likewise tend to come loose.
Some of the problems of foil regenerators are met by using a photoetched sculpted foil regenerator disclosed in U.S. Pat. No. 5,429,177, which allows cross-flows through perforations in the layers of foil. However, it is difficult to make regenerator foil in lengths exceeding about 1 meter by batch processes of photoetching and prohibitively expensive to make it in small quantities in continuous form. For best performance, all regenerator foil should be of the same density. However, it is difficult to make large pieces of phototetched regenerator foil of uniform density. If a spiral-wrapped foil regenerator requires more foil than can be etched in a single strip, then two or more strips must be spliced end-to-end. Splicing foils end-to-end is difficult because the foils are thin and delicate, thus difficult to align with the required precision. No fully satisfactory, inexpensive method of splicing has been demonstrated. Stringent requirements with respect to outgassing limit the bonding materials that can be used to join the ends of a foil strip to be used in a cryocooler application.
Pulse tube refrigerators are simpler and potentially more useful and reliable than Stirling cycle or Gifford-McMahon refrigerators. Their pulse tubes need not be perfectly round, which makes them less expensive to construct. However, in the prior art linear orifice pulse tube refrigerator shown in FIG. 3A, the cold heat exchanger is located between the pulse tube and the regenerator. The warm heat exchanger and aftercooler are at the far ends of the pulse tube and regenerator, respectively, leaving the cold part in the middle of the assembly. That arrangement is awkward for many potential cooling applications.
The geometry of the pulse tube refrigerator can be improved by placing the pulse tube inside the regenerator to create a coaxial pulse tube refrigerator according to prior art as shown in FIG. 3B. A coaxial pulse tube refrigerator is similar in shape to the cold finger of a Stirling cycle or Gifford-McMahon cryocooler. That similarity of form and function facilitates substitution of pulse tube refrigerators for Stirling cycle and Gifford-McMahon refrigerators in their many applications.
A disadvantage of coaxial pulse tube refrigerators is that they are hard to build. A major problem with construction of co-axial pulse tubes is in installing a regenerator in the annulus between the outer housing and the smaller pulse tube inside it. The regenerator must fill the annulus completely, with material of uniform geometry, in order to obtain satisfactory thermodynamic performance.
According to prior art, the housing and pulse tube of a coaxial pulse tube refrigerator are machined from tubing or bar stock to appropriate wall thickness. The pulse tube may be fabricated from insulating material to limit cross-flows of heat between pulse tube and regenerator, which typically display different temperature profiles over their lengths.
If the pulse tube and housing are assembled first, the regenerator material must be stuffed into the annulus afterwards. If the regenerator is to be made of stacked screens, they must be cut with accurate center holes and threaded over the pulse tube before being carefully and uniformly forced into the annulus between housing and pulse tube. Alternatively, the regenerator may be assembled first in its housing; the pulse tube must then be forced through the holes in the screens, which is difficult to accomplish without damage. In either case, preparing the screens with accurate center holes is difficult and expensive.
If the regenerator material is random fiber, it must be packed uniformly in a deep, narrow annular channel between housing and pulse tube. That requires painstaking labor, and it is difficult if not impossible to inspect the resulting regenerator for uniformity.
If the regenerator material is foil, its uniformity may be insured by inspection before it is installed. A spiral strip of foil may be wrapped around the pulse tube before the pulse tube is inserted into the housing. However, foil sufficiently fine to be effective regenerator material for cryocooler applications is delicate and the friction between the outer layer of foil and the inner wall of the housing can damage the foil if the fit is tight. If, however, the outer diameter of the foil regenerator is small enough to allow it to be inserted into the housing without risk of damage, it is difficult to obtain the tight fit necessary to eliminate preferential flow paths that seriously impair performance. This problem is exacerbated by the unevenness of the diameter of the wrapped foil due to the discontinuities at the inner and outer ends of the foil strip.
Several objects and advantages of this invention are:
(1) To provide a high performance foil regenerator for use in gas cycle machines.
(2) To provide easily-fabricated elements from which foil regenerators may be assembled.
(3) To provide an inexpensive, easily-assembled foil regenerator.
(4) To provide a foil regenerator with self-adjusting elements that insure a tight fit to housing walls.
(5) To provide a method of installing high-performance annular foil regenerators for gas cycle machinery.
(6) To provide an easily-fabricated pulse tube for use in a coaxial pulse tube refrigerator.
(7) To provide a coaxial pulse tube refrigerator with a tightly-fitted foil regenerator.
(8) To provide an inexpensive, easily-assembled coaxial pulse tube refrigerator.
(9) To provide a high performance foil regenerator for use in a coaxial pulse tube refrigerator.